1. Field of the Invention
The present invention relates generally to methods of oil recovery and more particularly to the use of alkylene carbonates in combination with a gas in methods that improve oil recovery from subterranean formations.
2. Background of the Invention
As the demand for fossil fuels and the market price of crude oil both increase there is a corresponding desire to produce more oil from maturing reservoirs. These efforts are referred to as Enhanced Oil Recovery (EOR) or Improved Oil Recovery (IOR). One form of EOR is the introduction of carbon dioxide at high pressures into a reservoir via injection wells. This form of EOR may increase production by elevating the reservoir pressure and by increasing the mobility of the oil through the reservoir. It is believed that carbon dioxide behaves like a solvent that swells the crude oil and decreases its viscosity.
However, crude oil is a complex mixture that contains low molecular weight, non-polar light ends and colloidal type dispersions of asphaltenes and other heavy polar materials. Only the light components of the crude oil typically interact favorably with carbon dioxide. As the light ends are removed by the carbon dioxide, the colloidal heavies typically become unstable and precipitate onto the formation. This precipitation becomes problematic as the precipitates may clog the pores and channels such that even the lighter hydrocarbons become trapped and unrecoverable from a reservoir.
Carbon dioxide has been injected into petroleum reservoirs in combination with surfactants such that foams are produced. These foams may improve the removal of asphaltenes and other heavy polar materials. However, the surfactants can be expensive, may adsorb onto the formation, and may increase the difficulty of separating the oil from water at the production well. The surfactants also are not believed to enhance the mobility of the oil by swelling it and making it less viscous.
Co-solvents have been proposed for use with carbon dioxide for enhanced oil recovery. Gases such as methane and butane will improve the oil properties, but because of their low polarity they mostly affect the light ends. Xylene has been used to remove asphaltenes from well bores. In the field, the use of aromatic solvents is not practical due to cost, safety and environmental issues. For example, toluene is a great solvent for the heavies found in crude oil but it is very volatile and has a low flash point and would require explosion proof equipment and special precautions for handling. Also toluene and similar aromatic solvents are not considered “green” and not likely to be used because of negative environmental effects.